The present invention relates to printing with a drop-on-demand ink jet print head wherein ink drops of varying sizes are generated. In addition to other applications, the present invention is particularly useful in grey scale or half-tone printing in which ink drop size is selectively varied during printing.
Ink jet printers, and in particular drop-on-demand ink jet printers having print heads with acoustic drivers for ink drop formation are well known in the art. The principle behind an impulse ink jet of this type is the generation of a pressure wave in an ink chamber and subsequent emission of ink droplets from the ink chamber through a nozzle orifice as a result of the pressure wave. A wide variety of acoustic drivers are employed in ink jet print heads of this type. For example, the drivers may consist of a transducer formed by a piezoceramic material bonded to a thin diaphragm. In response to an applied voltage, the piezoelectric ceramic deforms causing the diaphragm to displace ink in the ink chamber and causes a pressure wave and the flow of ink through one or more nozzles. Piezoelectric drivers may be of any suitable shape such as circular, polygonal, cylindrical, annular-cylindrical, etc. In addition, piezoelectric drivers may be operated in various modes of deflection, such as in the bending mode, shear mode, and longitudinal mode. Other types of acoustic drivers for generating pressure waves in ink include heater-bubble source drivers (so called bubble or thermal ink jets) and electromagnet-solenoid drivers. In general, it is desirable in an ink jet print head to employ a geometry that permits multiple nozzles to be positioned in a densely packed array with each nozzle being driven by an associated acoustic driver.
The prior art has also recognized that advantages may arise from printing with ink drops of selectively varying volume. For example, drop volume can be selected to provide optimum spot size to effectively produce high resolution printing. Also, by using only larger drops, a draft-mode print quality can be chosen. Such printers are also useful in applications requiring half-tone images, such as involving the control of color saturation, hue and lightness.
U.S. Pat. No. 4,513,299 of Lee, et al. describes one approach for achieving variations in ink drop size. In this approach, an electromechanical transducer is coupled to an ink chamber and is driven by one or more electrical drive signals of the same polarity which are each separated by a fixed time delay. This time delay is short with respect to the drop-on-demand drop production rate. Each electrical drive signal ejects a predetermined volume of ink with the ejected volumes of ink merging to form a single drop. An increase in the number of electrical drive signals between the formation and ejection of a drop causes an increase in the drop volume. This patent mentions that the various sized drops travel at a constant velocity to the print medium. This patent also recognizes that, because the print head is moving at a constant velocity during printing, any variation in drop velocity would cause displacement of the drops on the print medium from their desired position, and would degrade the print quality. However, inasmuch as all of the energy for drop formation and ejection results from the drive pulse supplied to the transducer, the variation in drop size is somewhat limited, the velocity of individual drops is limited, and some variation in the travel time to paper would tend to occur. In addition, the capacity of the ink jet to produce large ink drops using a large number of successive pulses limits the maximum rate of drop ejection. U.S. Pat. No. 4,491,851 of Mizuno, et al. illustrates another approach in which successive drive pulses are used to generate ink drops of varying sizes.
U.S. Pat. No. 4,561,025 of Tsuzuki describes another printer for printing half-tone images with ink drops or dots of varying sizes. The diameter of each dot is controlled by controlling the energy content of the driving pulse which causes the dot, for example, by varying the amplitude or pulse width of the driving pulse.
U.S. Pat. No. 4,563,689 of Murakimi, et al. discloses still another approach for achieving half-tone printing. In this approach, a preceding pulse is applied to an electromechanical transducer prior to a main pulse. The preceding pulse is described as a voltage pulse that is applied to a piezoelectric transducer in order to oscillate ink in the nozzle. The preceding pulse controls the position of the ink meniscus in the nozzle and thereby the ink drop size. In FIGS. 4 and 8, of this patent, the preceding and main pulses are of the same polarity. In FIGS. 9 and 11, of this patent, these pulses are of opposite polarity. This patent also mentions the control of ink drop size by changing the voltage and/or the pulse duration of the preceding pulse and the time interval between the application of the preceding pulse and the main pulse.
U.S. Pat. No. 4,403,223 of Tsuzuki, et al. describes a drop-on-demand type ink jet printer in which a driving pulse is applied to a piezoelectric transducer to cause the ejection of a drop of ink from a nozzle. The drop size is varied by controlling the energy content of the applied driving pulse for purposes of achieving half-tone printing. The ejected ink drops pass between charging electrodes and are charged by a voltage which is applied as the drops are ejected from the nozzle. This charging voltage varies as a function of the energy content of the driving pulses. In the embodiment of FIG. 10 of this patent, the charged ink drops pass between deflection plates which generate a field oriented transversely to the direction of drop travel for purposes of altering the flight path of the drops. In the FIG. 1 form of the apparatus, the charged drops pass between a pair of plates 40 and a pair of plates 60, with the deflection plates positioned between plates 60 and plates 40. The plates 40 and 60 establish an electric field oriented in the direction of travel of the ink drops for purposes of accelerating the drops.
The Tsuzuki, et al. patent requires relatively complex driving circuits inasmuch as the charging voltage is varied with variations in the driving pulse. In addition, the use of deflection voltages also adds to the complexities of this device.
Although these prior art devices are known, a need exists for an improved ink jet printer which is capable of effectively achieving half-tone or grey scale printing using a range of ink drop sizes and without requiring complex field switching or time delay circuitry.